HR: 14:40h
AN: T23F-05 INVITED     [Abstracts]
TI: Towards a complete in situ section of upper oceanic crust formed at a superfast spreading rate: Deep Drilling in Hole 1256D
AU: Teagle, D A
EM: dat@noc.soton.ac.uk
AF: University of Southampton, School of Ocean and Earth Science, National Oceanography Centre, Southampton, SO14-3ZH United Kingdom
AU: Umino, S
EM: sesumin@ipc.shizuoka.ac.jp
AF: Shizuoka University, Department of Biology and Geosciences, Shizuoka, 422-8529 Japan
AU: Alt, J C
EM: jalt@umich.edu
AF: University of Michigan, Department of Geological Sciences, Ann Arbor, MI 48109 United States
AU: Miyashita, S
EM: miyashit@geo.sc.niigata-u.ac.jp
AF: Niigata University, Department of Geology, Niigata, 950-2181 Japan
AU: Wilson, D S
EM: dwilson@geol.ucsb.edu
AF: University of California, Santa Barbara, Department of Geological Sciences, Santa Barbara, CA 93016 United States
AU: Banerjee, N
EM: banerjee@iodp.tamu.edu
AF: Texas A&M University, Integrated Ocean Drilling Program, College Station, TX 77845 United States
AB: The Superfast Spreading Rate Crust mission is a multileg program to drill, for the first time, a complete section of the upper oceanic crust from extrusive lavas, through the dikes, and into the underlying gabbros. Hole 1256D was initiated on ODP Leg 206 in the eastern equatorial Pacific and is drilled into 15 Ma crust that formed at the East Pacific Rise during a period of superfast spreading (>200 mm/a). This site is chosen to exploit the inverse relationship between spreading rate and the depth to axial low velocity zones, thought to be magma chambers now frozen as gabbros, observed from seismic experiments. IODP Expedition 309 successfully deepened Hole 1256D to a total depth of 1255 meters below seafloor (mbsf, 1005 m sub-basement) having penetrated through >800 m of extrusive N-MOR basalts and entered a region dominated by intrusive rocks with numerous subvertical chilled margins. The uppermost crust at Site 1256 comprises a >74 m-thick ponded lava flow overlying massive, sheet and minor pillow flows, some of which exhibit inflation structures requiring eruption onto a subhorizontal surface. This suggests a total thickness of off-axis lavas of 284 m. Sheet and massive lava flows make up the remaining extrusive section (534 to 1004 mbsf) above subvertical cataclastic zones, intrusive contacts and spectacular mineralized breccias denoting a lithologic transition zone. The extrusive lavas are less hydrothermally altered than other basement sites (e.g., Sites 417/418, Holes 504B, 896A) and there is no systematic change with depth from oxidizing to reducing seawater alteration. Instead, oxidizing alteration occurs irregularly most commonly associated with steeply dipping vein networks. Below 1061 mbsf, sheeted intrusives are dominated by massive basalts, some with doleritic textures, cross-cut by numerous subvertical dikes commonly with brecciated and mineralized chilled margins. These rocks are altered under greenschist facies hydrothermal conditions, and have significantly higher thermal conductivities and p-wave velocities. IODP Expedition 309 exited Hole 1256D cleanly and the hole is in excellent condition and ready for deepening. At 1255 mbsf Hole1256D is tantalizingly close to the minimum estimated depth for the frozen axial magma chamber predicted to be 1275 to 1525 mbsf. IODP Expedition 312 will return to this site in late 2005 and despite the gruelling 15 m per day pace of advance and assuming further benign drilling conditions, is set to deepen Hole 1256D by a further 500 m. The total depth would then be well beyond where geophysical interpretations predict gabbros to occur.
UR: http://iodp.tamu.edu/scienceops/expeditions/exp309.html
DE: 8416 Mid-oceanic ridge processes (1032, 3614)
DE: 8424 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8135)
DE: 8427 Subaqueous volcanism
SC: Tectonophysics [T]
MN: Fall Meeting 2005